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We ask the experts to settle common questions we've all wondered about.

QUESTION: Is there any convincing evidence that taking herbs or supplements can reduce your chances of catching a cold or flu?

ANSWER: This time of year, you will no doubt see lots of advertising for natural-health products that claim to help you avoid catching the common cold. Most products contain ingredients such as vitamin C, echinacea, garlic, ginseng and zinc, which are believed to stimulate your immune system and protect you from some respiratory viruses.

The product with the best data to support its claim is Cold-FX, which is made from a North American ginseng extract. In randomized, placebo-controlled clinical trials, the manufacturers found that healthy adults and seniors who took Cold-FX twice a day during the winter were about 25 per cent less likely to catch a cold, and that the colds they did catch lasted about 80 per cent as long. The effectiveness in children has not yet been tested.

There has been a great deal of research into the potential benefits of taking vitamin C. People who take any amount of vitamin C daily are just as likely to get a cold as those who do not. However, people who are taking more than 200 milligrams of vitamin C a day have colds that last a bit less time (8 per cent less) than people who don't take vitamin C.

The studies on echinacea and zinc preparations are contradictory. Some suggest that some preparations of both products might have an effect, but just as many studies fail to show even a trend toward an effect. The most important recommendation is that consumers be aware that available echinacea and zinc products differ greatly, and that the overwhelming majority of these products have not been tested in clinical trials.

Generally, these products are thought to be relatively safe, but allergic reactions and rashes can occur, and one homeopathic intranasal zinc preparation marketed in the United States was removed from the market in 2009 because its use was associated with permanent loss of smell. The safety data for long-term use for all preparations is limited.

There are a few studies of other natural health products, but none of these products is currently sold in Canada. Fortunately, more people are starting to pay attention to the burden of common colds, and the potential for naturally-derived products to reduce symptoms. As a result, we can expect to see many more studies over the next few years.

It is important to remember that natural products are naturally-derived chemicals. They should always be taken in their recommended dosages. You should read the label first for possible interactions with other medications you are taking, and it is a good idea to check with your health-care provider before starting any new medication or supplement.

While medication, whether prescription or non-prescription, may seem a relatively expensive and not very effective way to reduce your chances of getting acute respiratory infections, there are some simple ways that don't cost a lot and that are more effective in keeping you and your loved ones healthy this winter.

Most important, have everyone in your family vaccinated for the flu. Protecting yourself with a flu shot means one-third fewer coughs and colds overall, and stops you from passing the most dangerous of them, influenza, to other people. Similarly, washing your hands (or cleaning them with an alcohol gel) at least five times a day prevents about the same number of colds. Being careful to cough or sneeze into your sleeve if you don't have a tissue and to stay home when you get sick means that your co-workers and friends will be protected.

Dr. Allison McGeer is the director of infection control at Mount Sinai Hospital in Toronto. If you have a health question, send it to